In the past, in the case of a continuous annealing furnace for annealing a steel strip, for example, when operation is resumed after the furnace has been exposed to atmospheric air or when atmospheric air is mixed in a furnace atmosphere, a method for replacing the furnace atmosphere with a non-oxidizing gas, in which a non-oxidizing gas such as an inert gas is fed into the furnace as a replacing gas for the furnace atmosphere at the same time as the gas in the furnace is discharged while moisture in the furnace is vaporized by increasing a furnace temperature, has been widely used in order to decrease moisture and oxygen concentration in the furnace.
However, in the case of such a conventional method, since it takes a long time to decrease moisture content and oxygen concentration of the furnace atmosphere to specified levels which are appropriate for a regular operation, and since the furnace cannot be operated during all that time, there is a problem in that there is a significant decrease in productivity.
In addition, nowadays, in the fields of, for example, automobile, domestic electric appliance, and building material industries, there is an increasing demand for high-strength steel (high-tension material) capable of contributing to the weight reduction and the like of structural materials. In the case of a technique using this high-tension material, it is indicated that it is possible to manufacture a high-strength steel strip excellent in terms of stretch flangeability by adding Si in steel. In addition, in the case of technique using this high-tension material, it is indicated that it is possible to provide a steel strip excellent in terms of ductility owing to a tendency for a retained γ phase to be formed by containing Si and Al.
However, in the case of a high-strength cold-rolled steel strip containing easily oxidized chemical elements such as Si and Mn, there is a problem in that these easily oxidized chemical elements are concentrated in the surface of the steel strip during annealing and oxides of, for example, Si and Mn are formed, which results in surface appearance defects or defects in a chemical conversion treatment such as a phosphating treatment.
In the case of a galvanized steel strip, when it contains easily oxidized chemical elements such as Si and Mn, there is a problem in that these easily oxidized chemical elements are concentrated in the surface of the steel strip during annealing and oxides of, for example, Si and Mn are formed, which results in coating defects due to a decrease in zinc coatability or results in a decrease in alloying speed at the time when an alloying treatment is performed after a plating treatment has been performed. In particular, in the case Si, when an oxide film of SiO2 is formed on the surface of a steel strip, Si causes a significant decrease in wettability between the steel strip and molten plating metal, and the oxide film of SiO2 becomes a barrier to diffusion between the base steel and plating metal when an alloying treatment is performed. Therefore, Si particularly tends to cause problems by decreasing zinc coatability and alloying treatment performance.
As a method for preventing these problems, consideration is given to controlling the oxygen potential in an annealing atmosphere.
Patent Literature 1 discloses an example of a method for increasing the oxygen potential in which the dew point of the latter part of a heating zone and a soaking zone is controlled to be high, that is, −30° C. or higher. This method can be expected to be effective to some extent and has an advantage that the dew point can be controlled to be high in an easy industrial manner. However, this method has a disadvantage that, with this method, it is not easy to manufacture some steel grades (such as Ti-based IF steel) for which an operation in an atmosphere having a high dew point is not desirable. This is because it takes a very long time to control the dew point of an annealing atmosphere to be low once the dew point has been controlled to be high. In addition, since an oxidizing furnace atmosphere is used in this method, there is a problem in that there are pickup defects due to oxides sticking to rolls in the furnace and there is a problem in that there is furnace wall damage in the case where there is a control error.
As another method, consideration is given to controlling the oxygen potential to be low. However, in the case of a large-scale continuous annealing furnace which is installed in a CGL (continuous galvanizing line) or a CAL (continuous annealing line), since Si, Mn and the like are significantly easily oxidized, it has been thought that it is very difficult to control the dew point of the atmosphere to be normally low so that there is a good effect for preventing Si, Mn, and the like from being oxidized, that is, −40° C. or lower.
Techniques with which an annealing atmosphere having a low dew point can be efficiently achieved are disclosed by, for example, Patent Literature 2 and Patent Literature 3. Since these techniques are intended for comparatively small-scale furnaces of a one-pass vertical type, no consideration is given to an application to furnaces of a multi-pass vertical type such as a CGL and a CAL. Therefore, there is significantly high risk with using these techniques in that it may be impossible to efficiently decrease the dew point.